Many refinery and petrochemical processes involve separation of vapor from liquid phases, and subsequent neutralization or removal of acidic halogen species in either the vapor phase or the liquid phase in downstream processing steps. Several refinery processes (reforming and isomerization) are reviewed below, including a discussion as to why acidic halides are present in such streams, the difficulties caused by materials such as chlorides, and the problems of removing such acidic materials.
Catalytic reforming, using Pt based reforming catalyst, is one of the most important refinery processes in the world. Most refineries have a catalytic reformer, which converts naphtha fractions into high octane reformate.
Reformers come in many types and sizes--from 2000 BPD fixed bed units to moving or swing bed units processing more than 50,000 BPD. Reformers are available with fixed bed reactors, swing bed reactors, or moving bed reactors. Many new units are moving bed reactors, available from UOP, Inc, Des Plaines, Ill.
Reformers generally use mono-metallic catalysts (Pt on a support such as alumina) or bi-metallic catalyst (Pt-Re on a support). Other combinations of Pt and other metals are known.
All reforming catalysts are believed to contain halogen, almost invariably chlorine. Chlorine is now ubiquitous in catalytic reforming. Chloroplatinic acid may be used in the impregnation solution forming the catalyst. Some refiners may add chlorine compounds during normal operation.
One major oil company developed a Pt reforming catalyst regeneration or "rejuvenation" procedure which conducted at least some portions of the regeneration in the presence of one or more chlorine compounds. The procedure was believed originally developed for swing reactor systems which were regenerated every day or so, but this regeneration method, or some variant of it, was eventually used in semi-regenerative reformers and in moving bed reformers.
All of this chlorine can, and does, find its way into gas and liquid products from the reformer. Based on a review of several decades of The Oil and Gas Journal, the key to successful catalytic reforming is lots of chloride. For decades refiners have talked about the problems of getting enough chlorides into the system, and dealing with the chlorides in the vapor and liquid products from the reformer.
It should be clarified at this point that while most reformers use chlorines as a catalyst component, some may use other halogens, such as F or I, but C1 is the halogen of choice, so hereafter chlorine and its reaction or degradation products will be referred to rather than halogens in general.
In 1977 there was talk of the need for heat, chloride and moisture to redistribute platinum.
In 1980 there was a discussion of deposits of ammonium chloride in catalytic reforming compressor internals.
In 1985 there was discussion of the need for, and difficulty of maintaining, 1.0 wt % chloride on bimetallic catalyst between regenerations. It was suggested to "come out on the high side on chloride. "
In Alumina adsorbents effectively remove HCl from reformer H2 gas stream, Janke et al, Oil and Gas Journal, May 12, 1986, page 64, talked about controlled injection of organic chloride at the reformer reactor inlet, and the mischief caused by all this chloride. The problem was worse with continuous catalytic reforming processes, which were reported "to require higher levels of chloride addition for regeneration . . ." The solution proposed was use of alumina adsorbents to remove the HC.sub.1 from the net off gas. This article is incorporated by reference.
In Apr. 1, 1994 there was a discussion of the problem of corrosion in fired heaters due to chloride in the hydrogen from the reforming unit. The proposed solution was to install alumina treaters.
All of our refineries have some problems with chlorides in reformate. Several of them have installed alumina treaters to remove chlorides from the entire recycle gas stream.
The problem is not limited to reformers. Similar problems occur in some isomerization units, and may occur in other units which are dry and use a chloride containing catalyst.
The conditions which lead to chloride problems are catalysts which contain, or reaction conditions which require, chlorine compounds, and reactants which are dry enough that no separate aqueous phase forms in the vapor/liquid separator downstream of the reactor. Essentially all Pt reformers meet these conditions, and many isomerization and other processing units meet these conditions.
The situation could be summarized as follows for Pt reformers. Although refiners may use different reforming catalysts, all the catalysts seem to contain chlorine. There is enough chlorine either present in the virgin catalyst, or from chlorine addition during reformer operation, or from chlorine added during the catalyst regeneration, so that chlorine compounds appear in all the product streams coming from the reformer. Both vapor and liquid products have chlorine compounds, usually referred to as chlorides in most refineries, and these cause many problems.
The raw liquid reformate has chlorides. The net hydrogen gas make has chlorides. When raw reformate is fractionated, usually in a debutanizer, the overhead fractions contain chlorine compounds, unless the chlorides form salts which come out in the debutanizer and plug it.
To solve the problem of removing chlorides from gas streams, refiners have generally used beds of solid adsorbents, such as alumina impregnated with an alkaline material such as NaOH. Such approaches are discussed in the 1994 and 1986 OGJ articles discussed above, and are used commercially. While these approaches work, there are problems associated with the use of such alumina beds. The problems can include one or more of: cost, catalytic activity, regeneration and disposal.
Alumina beds are relatively costly, in terms of the amount of active ingredient present. The alumina typically contains 5 to 10 wt % caustic. Alumina costs more than caustic, and the alumina primarily serves as a support, but one which unfortunately is not always inert.
Alumina beds can exhibit catalytic activity. When alumina beds are used to remove chlorides from flowing vapor streams, aluminum chloride can form, and cause catalytic reactions which convert some of the hydrocarbon vapor species into a much higher molecular weight material. In some units, the gas is turned to goo, at least enough is formed that the effectiveness of the alumina bed is much impaired. This heavy viscous material must be removed to "regenerate" the alumina bed, so that it may be used to absorb additional amounts of chlorides or other acidic components from the flowing gas stream. Steam stripping will usually "regenerate" such a bed.
Disposal of solid adsorbents can be a serious waste management problem. Solid bed adsorbents must eventually be retired and frequently contains too much hydrocarbon to permit dumping in a landfill. The adsorbent bed may be steam stripped as a prelude to disposal. The resulting water/hydrocarbon product must be stripped to remove benzene from the waste water. The benzene and lighter hydrocarbons removed from the waste water are usually incinerated, and some of the chlorides may eventually end up as HCl, produced during the incineration of the benzene rich hydrocarbon phase.
I studied the problem of chloride removal from reformer vapor streams and realized that many of the problems could be overcome by a different approach.
Based on experimental work, I found that chlorides could be efficiently removed from a vapor stream in the presence of large amounts of liquid hydrocarbon. It was also possible to remove chlorides from hydrocarbon liquid, in the presence of large amounts of vapor.
By neutralizing or chemical reacting acidic halides at the same time as, and in the same vessel performing, vapor/liquid separation, it was possible to efficiently treat one, or the other, or preferably both streams at the same time. Not only was the chloride removal unexpectedly efficient, it also avoided most or all of the capital expense of a separate treater vessel.
Thus to remove chlorides from reformer recycle gas and reduce the amount of chlorides in reformate, I let a vapor/liquid separator associated with the reformer perform both the job of chloride removal and of vapor/liquid separation.
This approach not only saves money, it overcomes one difficulty associated with alumina treaters and gas streams. The difficulty is the polymer or goo that forms when chlorides in the gas exhaust enough caustic to form some AlCl3 catalyst from the alumina support used. Until now, there was no good way to remove this material, and some refiners had to provide swing bed treaters and go through troublesome stripping procedures to get the polymer off.
In my process, the polymer formed in the alumina treaters may be continuously washed off the alumina treater by the large amounts of reformate present in such separators. The minuscule amounts of high molecular weight material formed by the alumina treater will not be troublesome in the reformate, indeed they will generally be washed off as soon as polymerization (or oligomerization) proceeds far enough to produce liquids. Thus the life of the alumina treater can be extended significantly.
In fact, according to stoichiometric calculations, the loading capacity of a typical alumina adsorber should be 10-12 g of chloride/100 g of alumina. In actual practice, the loading capacity is found to be around 5 or 6 g Cl/100 g alumina. The loading capacity of alumina treaters in actual commercial use in this service is believed reduced at least in part to the build up of polymeric materials which cover the adsorption sites.
While the alumina treaters will eventually become spent, they will usually not need hot inert stripping to remove viscous oils formed on them, and will therefore have a higher on stream time. Many refiners will also have the ability, or for minor capital cost may acquire the ability, to change out the alumina treaters without shutting down the unit.
In many reformers, there are two vapor/liquid separators, though usually only the first one is called a separator. The reactor effluent is cooled and separated in a first vapor liquid separator. The gas is compressed, cooled, and recontacted with reformate or liquid from the first vapor liquid separator in a recontacting drum. This recontacting drum is a second vapor liquid separator. Alumina treaters may be put in either separator, or both, and this double treatment may provide enough reserve treating capacity that the unit will not need to shut down for alumina treater replacement until the next scheduled refinery turnaround.
A refiner may also provide piping and valves so that, for a day or a few days, one or the other of the separators may be isolated from service so that the alumina treaters in a given separator may be replaced. Thus the two V/L separators, which are already present in most recently built reformers, can serve as "swing reactors", or at least can provide a way to isolate one of the separators for servicing. Temporarily removing, e.g., the recontacting drum from service would not shut down the reformer. It would merely increase the molecular weight of the recycle gas, putting more load on the recycle gas compressor. All that happens to the unit is that the recycle gas flow drops some, or catalyst aging rates increase some, and hydrogen purity of the recycle and off gas drops a little. Many reformers have run without ever having a recontacting drum, so the loss of one separator for a few days can be easily tolerated.
The commercially proven alumina treaters will be preferred by many refiners for use in the vapor/liquid separator. These devices are mechanically strong, and will not plug or add anything detrimental to the recycle gas. I have also developed other methods of neutralizing acidic halides when both large amounts of hydrocarbon liquid and vapor are present which are potentially longer lasting and less costly. These are suitable for use herein.